1. Field of the Invention
Example embodiments relate to a CMOS image sensor (CIS). More particularly, example embodiments relate to a high-resolution CIS circuit performing correlated double sampling.
2. Description of the Related Art
A conventional CMOS image sensor (CIS) fabricated by a conventional CMOS process may be operated with a voltage lower than the voltage required for operating a conventional charge-couple device (CCD) and thus, a conventional CMOS image sensor (CIS) generally has power consumption less than the conventional CCD. Furthermore, the CIS may be formed using a standardized CMOS fabrication process and thus, pixels of the CIS may be highly integrated.
FIG. 1 is a block diagram of a conventional CIS circuit 100. Referring to FIG. 1, the conventional CIS circuit 100 may include a plurality of pixels 110 through 160 and a plurality of bias circuits 170 and 180. Each of the pixels may be operated by a sensed image transfer signal TGi, a pixel select signal SELi and a pixel reset signal RGi, wherein i is an integer corresponding to one of 1 through N and N is a number of pixels in each pixel array.
The conventional CIS circuit 100 may have M pixel arrays each including N pixels that may be arranged in a column, wherein M is an integer. A first pixel array may output a first signal APS_OUT1 through output terminals of N pixels 110, 120 and 130. The voltage of the first signal APS_OUT1 may be controlled by the bias circuit 170 associated with the N pixels 110, 120 and 130. An Mth pixel array may output an Mth signal APS_OUTM through an output terminal of another pixel array, which may include N pixels 140, 150 and 160. The voltage of the Mth signal APS_OUTM may be controlled by the bias circuit 180 associated with the N pixels 140, 150 and 160. The M signals APS_OUT1 through APS_OUTM may have information about video signals output from the pixels included in the pixel arrays.
Each of the pixels may include an active pixel sensor (APS) that may sense a video signal and output an analog voltage signal corresponding to the sensed video signal. Thus, a conventional CIS circuit using an APS generally requires an analog-digital converter (ADC) for converting the analog signal into a digital signal. Conventional CIS circuits may use a single ADC system or a column ADC system.
A single ADC system may convert APS analog output signals of all columns into digital signals within a fixed period of time using a single ADC. A single ADC system may have a relatively small chip area because only a single ADC is used; however, the single ADC system generally requires a relatively large amount of power to perform a high-speed operation to convert all of the signals within the fixed period of time.
A column ADC system may use a plurality ADCs, each having a relatively simple structure and being associated with a column of pixels in the column ADC system. Accordingly, a column ADC system generally requires a relatively large chip area; however, the power consumption of the column ADC system is generally less than the power consumption of a comparable single ADC system. A column ADC system may perform correlated double sampling (CDS) for APS analog output signals of columns and may process sensed video signals using a voltage obtained from the CDS.
FIG. 2 is an example timing diagram of a CDS operation performed by a conventional CIS circuit. Referring to FIG. 2, if the pixel select signal SELi for selecting an ith pixel of a pixel array is enabled (e.g., a high level), the pixel reset signal RGi may be disabled (e.g., transitioned to a low level) so that a power supply voltage VDDA may be provided to the ith pixel. While the pixel select signal SELi is at a high level and the pixel reset signal RGi is at a low level, a sampling signal SAM may be enabled a first and second time. The initial voltage of the pixel corresponding to the pixel select signal SELi may be sensed in response to a first enabling pulse of the sampling signal SAM and a voltage varied according to the initial voltage and a voltage corresponding to an image sensed signal may be sensed in response to the second enabling pulse of the sampling signal SAM. The voltage corresponding to an image sensed signal may be output as an ith signal APS_OUTi. Accordingly, the sensed image transfer signal TGi may be enabled at a time between the first enabling pulse and the second enabling pulse of the sampling signal SAM.
A high-resolution CIS may be needed to satisfy demands for high-resolution images. To provide a high-resolution CIS, a large number of pixels sensing video signals may be desired and/or required. However, a high-resolution CIS is generally operated at the same frame rate as a low-resolution CIS. Accordingly, the number of pixels processed within a predetermined period of time is generally greater in a high-resolution CIS than in a low-resolution CIS. Accordingly, the time allocated for processing a single pixel signal is generally shorter in a high-resolution CIS than in a low-resolution CIS.
An APS analog output signal APS_OUT may have a low voltage at the instant of time when a conventional CDS operation starts and may be charged with a voltage corresponding to a reset level of a corresponding pixel during a reset sampling period. FIG. 2 illustrates APS_OUTi, which is the charge time allocated to set an initial voltage for a pixel, which may be necessary prior to sensing a voltage corresponding to an image sensed signal from the pixel.
Referring to FIG. 1, to allow the output signal APS_OUT1 of the pixel 110 to have a desired and/or predetermined initial voltage, a desired and/or predetermined quantity of current flows from the source of the power supply voltage VDDA. The size of a MOS transistor indicated by a dotted-line circle may determine the quantity of current supplied from the source of the power supply voltage VDDA. For example, as the size of the MOS transistor increases, a speed of setting the initial voltage generally increases.
However, the size of the MOS transistor is generally reduced to obtain a higher resolution. As the size of the MOS transistor decreases, a time required for supplying charges to a capacitor Cp of a signal line generally increases. Consequently, the amount a pixel size may be reduced in order to obtain a higher resolution may be limited.